The present invention relates in general to solar energy collectors and more particularly to a new and useful solar receiver which comprises a plurality of panels of vapor generating tubes and a plurality of panels of superheater tubes interspersed with the panels of vapor generating tubes.
Significant effort has been directed in recent years toward the development of a solar receiver suitable for power generation. One of the more promising concepts is that of a solar power tower with a central receiver located on a tower. Sunshine falling on an array of computer guided tracking mirrors known as heliostats is reflected toward the receiver and the incident energy is absorbed by a heat transport fluid flowing in the receiver. The thermal energy in the fluid may be used to run a turbine generator or to provide heat for industrial use.
The absorber surface of a central solar receiver usually consists of tubular assemblies of various shapes and arrangements. Often, flat panels are used which are made up of parallel tubes that are welded to headers at each end.
Two basic configurations for these central receivers have been proposed and built, namely the external type and the cavity type. In general, external receivers are smaller, lighter, and less costly but have a lower thermal efficiency than cavity receivers. The external receiver may have one of several different shapes such as the shape of a multi-panel polyhedron approximating a cylinder or a part thereof. Examples of the external type are found in U.S. Pat. No. 4,136,674 to Korr; U.S. Pat. No. 4,289,114 to Zadiraka; and U.S. Pat. No. 4,245,618 to Wiener. U.S. Pat. No. 4,164,123 to Smith discloses a cavity type receiver.
Water or steam is a preferred working fluid for solar receivers because power plant operators are familiar with water/steam equipment. Water or steam receivers are actually steam generators operating on solar energy. A steam generator consists usually of three major parts connected in series: a preheater comprised of a group of economizer tubes; an evaporator comprised of a group of steam generating tubes; and a superheater. Steam generators are discussed in Steam, its Generation and Use, 39th Ed., The Babcock and Wilcox Company, 1978.
In a steam generator that delivers steam to a turbine, it is important to maintain a constant steam temperature and pressure within close limits even when the power varies. In a conventional steam generator, the heat of the combustion gases at any load is usually provided proportionately to all of the tubes of the three major parts (preheater, evaporator and superheater) so the proportion of heat absorbed by each part remains essentially constant. Although steam generation techniques are generally well known, solar receivers operate in an environment which is different from that of fossil or nuclear steam generators. A solar receiver is exposed to daily cycling from zero to peak power with a multitude of fast variations in heat distribution due to cloud transients.
In known solar receiver designs, the panels of tubes are arranged at fixed locations on the receiver, the locations being based on clear day insolation conditions. When, during a cloud transient, the heliostat field is partially shaded, the heat distribution to the tube panels becomes unbalanced resulting in loss of control of the steam temperatures from the superheater tube panels. Because of the decreased power input to the receiver from insolation, steam generation is reduced but the heat flux from insolation at some receiver areas may remain high. If high heat flux onto some superheater tubes coincides with high steam temperature due to reduced steam flow therein, the superheater tube metals will become overheated and overstressed resulting eventually in tube failure.
The prior art has sought to alleviate the problems discussed above, at the expense of high pressure losses and less efficient steam generation, by such means as requiring extremely high fluid flows through all of the tubes to compensate for random high heat flux in some of them, increasing the size of the solar receiver, or defocusing most of the bright heliostats from the receiver resulting in little power generation during the transient conditions.
If steam generating tubes are positioned in front of superheating tube panels to act as screen tubes, a greater number of superheating tubes are required for the same amount of power output since the superheating tubes will be partially shaded. In order to allow insolation to be received by the superheating tube panels, the screen tubes are not membraned. Since membrane members as well as the tubes absorb insolation, the use of membrane members with the steam generating tubes as well as with the superheating tubes reduces the number of steam generating tubes that are required for the same power output. In addition, a conventional buckstay arrangement cannot be provided to maintain appropriate intertube spacing and inhibit vibration of the screen tubes since such an arrangement would be undesirably exposed to the insolation. Support bars may be welded to individual screen tubes respectively which bars extend through slots in webs of the corresponding superheating tube panels for attachment to a complex and expensive vibration support structure in back of the superheating tube panels which structure is in turn attached to the webs. However, such a structure is more expensive than a buckstay arrangement, and it is more desirable to provide an arrangement of the tubes to alleviate the insolation exposure problems and whereby a less expensive buckstay arrangement can be used to maintain panel shape and facilitate erection of the panels in a receiver having a minimum number of tubes for the same power output.